1. Field of the Invention
The present invention relates to a surface light source device, an optical member and a display device which can achieve a uniform in-plane distribution of luminance.
2. Description of Related Art
A direct-type surface light source device for use in a transmission type display device includes a light source and a plurality of optical sheets (optical films) for changing the travel direction of light from the light source. A light source, comprised of a plurality of light emitters arranged in parallel, is widely used in such a surface light source device. The plurality of optical sheets include a light diffusing sheet for diffusing light from the light source to shade (obscure) the image of the light source, and a light condensing sheet for condensing light in the front direction to enhance the front direction luminance.
An optical sheet, having linearly extending unit shaped elements (unit optical elements) arranged in a direction perpendicular to the longitudinal direction of the elements (so-called linear array), is widely used as a light condensing sheet (see e.g. WO 2007/094426A1). In particular, unit lenses, each having a shape corresponding to part of a circle or an ellipse in a section (main section) perpendicular to the longitudinal direction, are most commonly used as the unit shaped elements. Such an optical sheet can change the travel direction of light and adjust the angular distribution of luminance mainly in a plane along the arrangement direction of the unit shaped elements. Therefore, two such optical sheets are sometimes incorporated into a surface light source device such that the arrangement directions of their unit shaped elements are perpendicular to each other, as disclosed in WO 2007/094426A1. This enables adjustment of the angular distribution of luminance in two different directions.
In a surface light source device which uses a light source comprised of a plurality of light emitters, the luminance tends to be lower in an intermediate region between two adjacent light emitters. The above-described light condensing sheet, used in a surface light source device, also has a light diffusing function to reduce such in-plane variation of luminance. Practically, however, it has not been possible only with the use of the light diffusing function of a light condensing sheet to equalize the in-plane distribution of luminance and sufficiently obscure the image of light emitters. Therefore, a surface light source device generally has a light diffusing sheet disposed between a light condensing sheet and a light source. When the light diffusing sheet has a strong light diffusing effect, the light diffusing sheet can cause a problem, such as lowering of the efficiency in the use of source light. It is therefore highly desirable if an optical sheet, having a light condensing function, can effectively perform a light diffusing function, whereby it becomes possible to use a light diffusing sheet having a reduced light diffusing effect, or to even eliminate the use of a light diffusing sheet.